1. Field of the Invention
This invention concerns a process for regenerating waste oils of synthetic lubricants containing fluorine atom and, more specifically, it relates to a process for easily regenerating the waste oils of synthetic lubricants containing fluorine atom to a quality compatable with that of fresh oils.
2. Description of the Prior Art
Synthetic lubricants containing fluorine atom are those synthetic lubricants having the structure represented by the general formula: ##STR1##
It has been known that such lubricants exhibit excellent oxidation-resistance, chemical-resistance and heat-resistance, as well as being non-toxic in contrast to other lubricants. Further, the synthetic lubricants containing fluorine atom are also greatly different from other lubricants in the physical properties thereof. For instance, they have higher specific gravity compared with other lubricants and are not soluble in water and most organic solvents, as well as being highly stable chemically.
As described above, since the synthetic lubricants containing fluorine atom have such excellent characteristics as not found in other types of lubricants, they are frequently employed owing to their physical properties mainly in applications requiring heat resistance and oxidation resistance or in applications requiring stabilized lubricating performance for a long period of time.
In addition, the synthetic lubricants containing fluorine atom have recently been noted as materials for use in vacuum techniques and are widely employed as the lubricants, particularly, for vacuum pumps to be used in the electronic industry or the like in view of their foregoing properties. It has been recently recognized that the synthetic lubricants containing fluorine atom are quite superior to other types of lubricants with respect to those requirements where the lubricants to be used in the vacuum pumps must exhibit lubricating performance, oxidation resistance, heat resistance, chemical resistance, low vapor pressure, radiation resistance and non-toxicity. Particularly, because of excellent chemical stability, the synthetic lubricants containing fluorine atom are now widely utilized as the lubricants for vacuum pumps in the electronic industry in which dangerous gases which are highly corrosive and contain a lot of dust are often used.
However, even if the synthetic lubricants containing fluorine atom used in vacuum pumps have such a high chemical stability, they have to be finally discarded when they are contaminated and degraded, for instance, by incorporation of various materials discharged from a system to be evacuated, incorporation of abrasive powder of the vacuum pumps and denaturing of the synthetic lubricants containing fluorine atom themselves.
As described above, although the synthetic lubricants containing fluorine atom are chemically inactive and, accordingly, less degradable compared to the lubricants of petroleum origin and have advantageous characteristics as the lubricants for use in the vacuum pumps, contaminants incorporated therein are not dissolved. As a result, the contaminants mixed into the lubricants gradually accumulate. Consequently, in the synthetic lubricants containing fluorine atom, since incorporated contaminants are suspended in the lubricants, fresh lubricants which are initially transparent and viscous gradually become opaque as they are used and finally turn to waste oils with a brownblack appearance and increased viscosity. Generally, the need for replacing the lubricants used in vacuum pumps for the electronic industry, particularly, as used in the dry etching process employed in the production of semiconductor devices, has been judged on the basis of the exhaustion velocity of the vacuum pump. As the exhaustion velocity lowers, it becomes difficult to discharge contaminants produced from etching or the like and it causes unevenness in the etching result. For instance, in an etching step using a gas mixture of CF.sub.4 and O.sub.2, if it requires more than 13 sec after the starting of the vacuum pump to attain a vacuum of 0.2 Torr, which can initially be obtained within 9-10 sec by fresh oils, unevenness occurs in the etching result. In this state, the lubricants appear brown-black and opaque. If such degraded lubricants are used as they are for a long time continuously, it not only gives an undesirable effect to the etching process but also impairs lubricating performance due to the contaminants mixed in the lubricants upon actuation of the vacuum pump, whereby an undesirable effect is also given to the components of the pump (particularly, bearings and seals of the rotational shaft), to further worsen the function of the vacuum pump. As a result, the time required for attaining an intended vacuum degree is increased or the pressure attainable by the vacuum pump is lowered. Accordingly, lubricants which have turned to such a brown-black color can no longer be used and should be discarded as waste oils.
As is well-known, however, since the synthetic lubricants containing fluorine atom are chemically stable and non-flammable, it is impossible to treat the waste oils through burning or heat decomposition, and the disposal of the waste oils of synthetic lubricants containing fluorine atom provides a significant problem to be solved. Since the synthetic lubricants containing fluorine atom are much more expensive as compared with other synthetic lubricants, development of a process for regenerating the waste oils thereof has keenly be desired, particularly, in the electronic industry where the frequent exchange of lubricants containing fluorine atom is required.